Parts used in automobiles and OA equipment are becoming thinner and lighter rapidly. For example, attempts are being made in the automobile industry to manufacture large-sized parts typified by body panels such as a fender from resin compositions in order to reduce the weight of an automobile.
One of the resin compositions used in automobiles and OA equipment is a resin composition which comprises PC and PET (may be referred to as “PC/PET alloy” hereinafter). Since the PC/PET alloy has both the characteristic properties such as high impact resistance and stiffness of PC and the characteristic properties such as high chemical resistance of PET, it is a useful resin composition and various improvements on the composition are proposed.
For example, patent document 1 discloses a car side protector which is manufactured by injection molding a resin composition comprising PC, PET, an impact improving material and mica. The resin composition is obtained by making use of impact resistance and heat resistance which are the advantages of PC and improving the chemical resistance which is the weak point of PC with PET. However, the fluidity of the resin composition has room for improvement and a trouble may occur when a large-sized part is molded.
Patent document 2 proposes a resin composition which comprises PC and recycled PET. Although the resin composition has advantages such as impact resistance, stiffness and heat resistance derived from PC, the fluidity of the resin composition must be improved in order to mold a large-sized, thin and lightweight part. Since the resin composition comprises the recycled PET, it is unsatisfactory in terms of heat stability.
Like the above documents, various proposals have been made to improve the physical properties of a resin composition which essentially comprises PC and an aromatic polyester such as PET. For instance, patent document 3 discloses a resin composition which comprises PC, an aromatic polyester, a rubber-like elastic material and fine fibers surface treated with a silane-based compound to improve weld strength, stiffness and impact resistance. Patent document 4 discloses a resin composition which comprises PC, an aromatic polyester, wollastonite, an ethylene-ethyl acrylate copolymer and a copolymer wax of an α-olefin and maleic anhydride. The proposal of this document aims to improve the surface appearance, chemical resistance, impact resistance and stiffness of a molded product. Further, patent document 5 discloses a resin composition which comprises PC, an aromatic polyester and wollastonite having a specific shape. The proposal of this document aims to improve the impact resistance, recyclability and surface appearance of the resin composition. In addition, patent document 6 discloses a large-sized molded product obtained by injection compression molding a resin composition which comprises PC and an aromatic polyester. The proposal of this document aims to improve the dimensional stability and impact strength of the resin composition.
However, all of the above proposals aim to improve mechanical properties such as impact resistance and stiffness and thermal properties such as heat resistance, and the fluidity of a resin composition required to mold a large-sized thin part is not studied to the full.
As technology for improving the fluidity of a resin composition, patent document 7 discloses a resin composition which comprises a low-molecular weight polybutylene terephthalate, a high-molecular weight polyester and a specific phosphorus-containing compound. However, this technology cannot be applied to the PC/PET alloy which differs from the above resin composition in resin component. The resin composition has room for improvement in stiffness, impact resistance and heat resistance.
As a resin composition for car parts, patent document 8 discloses a resin composition which comprises PC, ABS and talc and has a specific linear thermal expansion coefficient, specific dart impact strength and a specific deflection temperature under load.
Although a large number of proposals have been made to improve the impact resistance, stiffness, heat resistance and heat stability of the PC/PET alloy as described above, a resin composition which has fluidity required for molding a large-sized part which must be thin and lightweight and chemical resistance required for molding a part which must have a stable coating appearance has not been proposed yet.
Meanwhile, sequential valve gating (to be referred to as “SVG method” hereinafter) for opening or closing a plurality of hot runner valves sequentially according to a program is suitable for the manufacture of large-sized molded products such as car parts (refer to non-patent document 1). Although SVG method may be used to mold a plurality of molded products having different shapes from a plurality of gates, when one molded product is molded from a plurality of gates, a high-quality large-sized resin molded product is obtained. In the latter case, after a molten resin from the previous gate passes, the gate is opened to fill a resin by carrying it on a flow of the molten resin. SVG method molding is generally called “cascade molding” in which the above operation is carried out stepwise by each gate to supply the molten resin. Since this cascade molding can suppress a weld line and increases the degree of freedom of the number of gates, it has an advantage that a large-sized molded product can be injection molded with relatively low clamping force. Therefore, a resin composition having excellent fluidity suitable for this molding method is desired.    (patent document 1) JP-A 4-224920    (patent document 2) JP-A 2003-128905    (patent document 3) JP-A 8-259789    (patent document 4) JP-A 9-012847    (patent document 5) JP-A 2002-265769    (patent document 6) JP-A 2003-171564    (patent document 7) JP-A 6-279664    (patent document 8) JP-A 2-294358    (non-patent document 1) Plastics Technology, December 2003, p. 38